1. Field of the Invention
This invention relates to a hydrostatic motor unit, comprising at least two hydraulic motors, at-least one of which has a variable capacity, whereby the hydraulic motors can be switched hydraulically in parallel, and can have output rotors that are coupled with each other.
2. Background Information
Such a motor unit designed to be used as the central propulsion unit of a work machine is described in DE 42 35 710 A1. This prior art motor unit consists of a high-speed axial piston motor with a variable capacity and, located axially parallel to it, an internally pressurized low-speed radial piston motor with a constant capacity. The output shaft of the axial piston motor has a gear wheel engaged with gear teeth attached to the outer periphery of the rotationally mounted housing of the radial piston motor. The speed difference between the higher-speed motor and the lower-speed motor is equalized by a step-down transmission between the gear wheel and the gear teeth.
When the two motors are switched in parallel, the two hydraulic motors, depending on their individual capacity, each have an output moment. The two output moments are output together as the total output moment. The radial piston motor can be disconnected by separation from the hydraulic circuit and pressurization of the piston toward the internal top dead center position. The hydraulic fluid released is available for the axial piston motor. The axial piston motor, immediately before the deactivation of the radial piston motor, is set to the minimum capacity. When the radial piston motor is deactivated, the axial piston motor must be very quickly reset to the maximum capacity, to be able to absorb the additional hydraulic fluid from the radial piston motor without a switching pulse (which corresponds to synchronization). By a subsequent reduction of the capacity, and at a reduced total output moment of the motor unit, the output speed of rotation of the motor unit can be increased further with respect to the hydraulic parallel operation of the two hydraulic motors, and the conversion range of the motor unit can be increased.
The drive system experiences an interruption in the traction produced at the moment of the deactivation of the radial piston motor and the reduction of the axial piston motor. This interruption is undesirable in certain applications. For example, such applications include traction drive systems for work machines such as fork lift trucks, etc. Due to the potential resulting interruption in the braking force, a motor unit of the prior art cannot be used in traction drive systems that operate in a closed circuit and which are also used for operating brakes.
On account of their large size and the effort and expense involved in their construction, the motor unit described above is also unsuitable for applications where space is limited, e.g., in wheel drive systems. In most such cases, variable high-speed axial piston motors are used, downstream of which a step-down gear train is located.
The prior art also discloses the use of low-speed radial piston motors as wheel drives (direct drive). Radial piston motors, however, do not have a variable capacity with a multiple-stroke construction, which significantly limits the types of potential applications. In a single-stroke form, of course, the capacity can be varied. Although in that case, such drive systems represent high-speed motors, which require a subsequent stepping down, if the objective is to have low output speeds with a high output moment.
The object of this invention is to make available a hydrostatic motor unit of the type described above that is easy to manufacture, compact and can be used in a wide variety of applications.